Ceramic-metal laminate

ABSTRACT

A ceramic-metal composite laminate capable of exposure to high temperature differentials without damage, consisting of an inner ceramic layer, an outer metal layer and an intermediate interface layer of a low modulus metallic low density structure having a high melting point. The ceramic layer is secured to the low modulus structure directly or through an intermediate ceramic-metal composite, and the outer metal layer is brazed to the intermediate low modulus layer. Thermal strains caused by a temperature differential between the inner and outer layers are taken up without harmful effect by the intermediate low modulus layer.

This is a continuation, of application Ser. No. 337,669, filed on Jan.7, 1982, now abandoned, which is a continuation of application Ser. No.939,888, filed on Sept. 5, 1978, now U.S. Pat. No. 4,338,380, which is adivision of application Ser. No. 674,047, filed on Apr. 5, 1978, nowU.S. Pat. No. 4,142,022.

BACKGROUND OF THE INVENTION

1. Field of the Invention

This invention relates to ceramic-metal laminates, and moreparticularly, to the method for making a laminate of this type whichenables thermal strains to be taken up without harmful effects and tothe article produced by the method.

2. Description of Prior Art

A number of methods are known in the prior art for joining a metalmember to a ceramic section. For example, U.S. Pat. No. 2,996,401 showsa method for use in electron tube manufacture where the surface of theceramic body is metallized with refractory metals and the metal memberis then brazed to the metallized coating. Another example, U.S. Pat. No.3,114,612, shows a ceramic laminate useful for high temperatureapplications where the ceramic is coated with a metallic bonding mediumand welded to a corrugated stainless steel sheet.

While these prior art methods are satisfactory in uses for which theywere designed, in high temperature operation under oxidizing conditionsand mechanical stress, such as encountered in gas turbine engines, therequired laminates must have the ability to withstand the substantialstrains due, in part, to extreme differences in amounts of thermalexpansion which are created during turbine's operation and in part dueto the thermal gradients across them. The prior art items tend to beanisotropic in their ability to absorb thermal strains and there is aneed for an attachment method that will respond to thermal strainselastically at moderately low stress levels in all directions.

SUMMARY OF THE INVENTION

Direct joining of ceramic materials to metallic materials is presentlylimited to materials having small differences in coefficient of thermalexpansion (0.5×10⁻⁶ in/in/°F.) and in the geometry of the structure (theceramic material must remain in compression). Differences in coefficientof thermal expansion (α) can be minimized by using a technique wherematerials with closely matched α's are provided adjacent to each otherforming a gradient of ceramic (α_(c)), cermets (α₁ . . . α_(n), whereinthe cermets are mixtures of powdered metals and ceramics varying indensity such that with sufficient thickness there can be an infinitenumber of layers, each having a slightly different α) and metal depictedthusly: ##STR1##

Unfortunately, this technique is severely limited to low temperature usebecause of the temperature limits imposed by: (1) relatively lowoxidation resistance of low thermal expansion alloys, and (2) widediversities of expansions at elevated temperatures of the metal, cermetsand ceramic materials.

The development of high temperature abradable gas path seals for use inturbine engines has necessitated the development of a method for makinga ceramic-metal laminate which is not limited by differences inexpansion rates or lack of oxidation resistance.

In such high thermal gradient conditions where the surface of theceramic experiences temperatures of 1000° F. to 3000° F. and there is atemperature gradient across the ceramic, the hot surface expands greaterthan the cooler surface. If this expansion is constrained as in theceramic-cermet-metal laminate excessive stresses are built up in theceramic material causing failure by thermally cracking. Thus, thislaminate is not acceptable where thermal gradients in excess of 500° F.to 1000° F. occur. For example, when the ceramic is alumina and themetal is a Ni-Al alloy, then a temperature gradient of 500° F. in such astructure would not perform properly.

Accordingly, the present invention comprises a ceramic layer; a threedimensional, flexible, resilient, low modulus, low density, metallicstructural interface secured to the ceramic layer; and a metal memberfastened to the low modulus metallic structure. Thermal strains causedby differences in the coefficients of thermal expansion of the metalmember and ceramic are absorbed by the low modulus material interfacewhich has sufficient tensile strength, resistance to oxidation at hightemperatures and resilient flexibility.

The principal object of this invention is to provide a ceramic-metallaminate which can be used in high temperature applications, especiallyseals for blades in gas turbine engines.

A further object is to provide a ceramic-metal laminate wherein thermalstrains caused by the different values of thermal expansion andcontraction of the ceramic and metal are taken up by a low modulusmetallic low density structure interface interposed between the ceramicand the metal laminates.

Another object is to provide a method for joining the ceramic to a lowmodulus metallic mat structure.

Still another object is to provide a method for joining the intermediateceramic-metallic composite to the low modulus metal mat stucture wherebythe tensile strength of the ceramic would not be exceeded at thejunction during thermal expansion.

A still further object is to provide a felted metal mat of high meltingmetal fibers as the low modulus porous resilient interface member.

Yet a further object is to provide a method for joining one face of themetal fiber mat to a metal structure by brazing, and the other face ofthe mat (or web) to a ceramic structural layer.

Another important object of this invention is the formation of analumino silicate material felted porous ceramic that preventsdevitrification of the quartz by the addition of a low expansion glassthereto.

Yet another object of this invention is to provide an attachmentinterface between ceramics and metals operating cyclically to extremetemperatures (either high or low) from ambient with high temperaturegradients across them which is essentially isotropic with regard to itsresiliency and low modulus characteristics.

Yet another object of this invention is to provide an attachmentinterface between ceramics and metals operating with high temperaturegradients between them that has a low thermal conductivity to minimizeheat losses.

Further objects will become apparent from the following detaileddescription of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a sectional view of the primary embodiment of this inventiondepicting the ceramic-resilient interface-metal composite.

FIG. 2 is another sectional view of the invention.

FIG. 3 is a sectional view of another embodiment of the invention.

FIG. 4 is an enlarged sectional view of a facet of the invention.

FIG. 5 is an enlarged sectional view of another facet of the invention.

FIG. 6 is a sectional view of an intermediate product of one of theembodiments of the invention.

FIG. 7 is a photo-macrograph of the primary embodiment at a 15Xmagnification.

DESCRIPTION OF THE INVENTION

In the embodiment shown in FIG. 1, a cross section view of an abradablehigh temperature seal 100 for a gas turbine engine is shown. Ceramicmember 1 may be made of high temperature ceramics such as alumina,stabilized cubic zirconia, magnesia, zircon (ZrO₂.SiO₂) fosterite(2MgO.SiO₂), mullite, mullite and quartz, aluminum di-boride, calcia,ytria, glass, silicon carbide, silicon nitride, alumino-borosilicate,etc., that have any desired thickness and degree of porosity such asneeded for high temperature abradable seals.

The ceramic member 1 has a very low coefficient of thermal expansiongenerally in the range of about 1×10⁻⁶ to 8×10⁻⁶ inches per inch perdegree F. Conversely, the metal base 3, that the ceramic member 1 isultimately joined to, has a very high coefficient of thermal expansionin the range of about 2×10⁻⁶ to 20×10⁻⁶ inch per inch per degree F. Inthe environment of a gas turbine engine, where the outer surface of theceramic member 1 is subjected to temperatures in the neighborhood of1800°-3600° F., while the exposed surface of the metal is subjected to atemperature range of only several hundred degrees F., a direct joiningof the two would cause immediate rupture of the ceramic due to thedifference in coefficients of expansion and to the effect of thetemperature gradient through the thickness of the ceramic. Thus, in thisinvention and the primary embodiment of this invention, a resilient, lowmodulus, elastic interface 2 is secured to both the ceramic member 1 andthe metal base 3 absorbing geometric differences caused by thevariations in thermal expansion of the two materials and by thetemperature gradient.

The interface 2 comprises a three dimensional, flexible, resilient, lowmodulus, low density, porous, high melting point metallic fiber web ormat structure such as described in detail in U.S. Pat. Nos. 3,469,297;3,505,038; or 3,127,668. Typical alloys used for the fibers of thisinterface are sold under the trademarks of Hastelloy X, Hoskins 875,Haynes 188, DH 242, as well as the nickel base super alloys and thequadrinary and quintinary alloys of iron, cobalt, nickel, chromium,aluminum, and yttrium (or the rare earths). Desirably, the porous web ormat structure has a density of approximately 35%, although dependingupon the particular application the web comprising the interface 2 canhave a density varying anywhere from 5 to 80%. It will be apparent thatthe exact alloy employed in making the mat will be dictated by thetemperature, oxidation, and stress conditions to be encountered in theultimate use.

One method of making the embodiment shown in FIG. 1 is to braze a webinterface 2 to the metal base 3 as shown at 20. The ceramic layer 1 isformed by plasma spraying the ceramic material onto the exposed face ofthe interface 2 wherein the ceramic material impregnates into thesurface of the web interface 2 bonding the ceramic mechanically to thefibers of the interface 2. Subsequently, additional plasma spraying ofthe ceramic will provide the desired thickness of the ceramic layer 1.The product thus produced is a ceramic-metal composite having aresilient interface so that when the metallic member expands due tothermal expansion a much greater amount than the expansion of theceramic material, the interface can absorb the different amounts ofthermal expansions of the two materials. Thus, a composite is providedwhere there is a high degree of thermal expansion mismatched between theceramic 1 and the adjoining metal 3, but able to remain intact overextreme thermal cyclings because of the ability of the metal webinterface 2 to absorb the differential expansion and the resultingthermal strain.

In FIG. 2, there is an enlarged view of the basic embodiment as shown inFIG. 1, wherein the metal fibers 4 of the web interface 2a are shown toprotrude into the ceramic surface up to approximately 1/4 the thicknessof the ceramic material 1a while the other surface of the interface 2ais brazed at 20a to the metal plate 3a. Here, the ceramic layer 1a hasembedded into its surface the metal felted mat interface 2a. The ceramicand metal alloy must be so selected so as to minimize the chemicalreaction between the metal fibers of the interface and the ceramicthereby providing primarily mechanical bonding between the two. Therceramic-metal interface composite portion may be formed by pressing themetal felt interface into a plastic mass of the ceramic material for adistance sufficient to insure a mechanical bond of sufficient strength;this being another method of joining the ceramic and the interface. Asmentioned above, about 1/4 of the thickness of the ceramic layer 1awould be sufficient to have the interface 2a embedded therein; however,this can be varied as may be required by the design. After the mat isembedded into the plastic ceramic, the thus formed composite is driedand fired. As mentioned earlier, the web interface can be attached tothe metal base first or may be attached to the metal base after theceramic has been joined to the interface, as desired.

In another embodiment of the invention, approximately 3/8ths of an inchU-shaped card wire staples are secured to a fabric base and forcedtherethrough in a generally upright position. A ceramic-water slurringmixture of ceramic fibers and/or powders is deposited on the fabric andconfined within the area of the metal staples. This initial material isthen sintered in a furnace in order to react the ceramic slurry to formthe desired ceramic material and at the same time mechanicallyincapsulate the wire staples. As shown in FIG. 6, the fabric 14a haswire staples 13a which are imbedded in the ceramic material 12a therebydefining a ceramic-wire layer 11a. When the ceramic is fired in thefurnace, the fabric layer 14a disintegrates, leaving the staples 13aimbedded in the ceramic 12a. The protruding staples of the layer 13a maybe bent flat on both surfaces for convenience. As with the metal fiberweb interface material, chemical reaction between the wire staples andthe ceramic material must be minimized, otherwise stresses resultingfrom the mismatch of the coefficients of expansion (α' s) would causecracking of the material. Chemical reaction between the metal and thecomposite would result in a strong bond between them which would promotedegradation in the ceramic and metal interfacial area and minimizemovement of the two.

As seen in FIG. 3, this ceramic layer with staples 11a therein issecured to a purely ceramic layer 1b by glass frit 23b by placing thetwo in a furnace at elevated temperatures for a short period of time. Ametal base 3b is brazed as shown at 20b to a porous web interface 2b.The ceramic laminate with the exposed staples is then spot brazed to theweb interface 2b at 10b. Since the fibers of the interface 2b are not100% dense, and since obviously the staples 13a of the ceramic layer arenot 100% dense, the brazing of the two may be 100%, but the total areaof the metal will not be greater than the metal density of the smallestmetal material. This type of composite also exhibits the samecharacteristics and desirability as the basic embodiment shown in FIG.1.

It should be noted that in FIGS. 4 and 5 the geometric bond between theceramic 1 and the fibers 4 of the interface 2 or the staples 13a promotea mechanical bond. This particular characteristic is extremely importantfor the operation of this material.

Besides being made from the metals listed for the interface, the staple13a may also be made from materials such as platinum, tungsten,molybdenum and the like depending upon the environment. The ceramicmaterials used in this invention are those commercially found availableas high temperature ceramics as well as the unexpected materials foundby us and described hereinafter.

In the use of ceramic materials for high temperature seals and gasturbines, and especially where the seals are abraded such as taught bythe prior art, for example in U.S. Pat. No. 3,880,550, a sinteredproduct of an alumino silicate ceramic consisting of mullite plus quartzas a high temperature material and insulation (above 2600° F.) is verylimited. The free quartz present in the available fibers (known by thetrade names of Fiberfrax and Kaowool) becomes brittle because the fusedquartz devitrifies and converts to cristobalite when exposed totemperatures of 1800° F. and over. It has been found that by using amixture of alumino-silicate fibers and low expansion glass fibers orpowders, which is subsequently sintered, a ceramic material may beformed to operate at temperatures in the range of 2200°-3000° F. (anincrease of well over 400° F. for known Fiberfrax). In sintering themixture, the glass surrounds the alumino-silicate and at the same timedissolves any free quartz and results in a mixture of mullite and glass.This new ceramic has been employed as one of the porous materials usedfor the ceramic portion of the composite material taught herein. Quitesurprisingly, this material was found to exhibit excellent hightemperature characteristics. In using a standard alumino-silicate,typically 35-55% SiO₂ and 45-65% Al₂ O₃ at temperatures above 1800° F.the fused quartz also devitrifies and forms cristobalite which severlyimbrittles the fibers and weakens the general product. By the additionof fibrous or powdered glass to the aluminum silicate the glass reactswith the quartz to form a new glass that will not devitrify. Three typesof fiber forming materials having different temperature ranges that,when subject to this glass powder-fiber technique, produce a much betterceramic are alumino-silicate, alumina, and zirconia.

The cobalt-base super alloy base 3 is shown in FIG. 7; a macrophotographat 15X. The metal web 2 is about 20% dense, made from Hoskins 875 alloyand brazed to the base 3. A ceramic layer 1 was plasma sprayed on theweb and embedded therein as may be seen in the macrophotograph. Theceramic layer 1 was composed of CaO, 4% by weight and ZrO₂ --96% byweight.

The following specific embodiments of the ceramic-interface-metalcomposites made in accordance with this invention should not beconstrued in any way to limit the scope contemplated by this invention.

EXAMPLE I

According to the teachings of U.S. Pat. No. 3,127,668 a felt web madefrom one-half inch kinked 5 mil wire of FeCrAlSi (Hoskins-875) metalalloy was sintered for 15 hours in a furnace vacuum of 10⁻⁵ torr and ata temperature of 2175° F. The web produced had an approximate 30%density. A metal base of a high temperature cobalt base alloy was brazedto the sintered web by exposing the web and the base metal to 2150° F.in a vacuum furnace for about 10 minutes. The zirconia, in atmosphere,was plazma sprayed onto the exposed web surface impregnating the web atleast 10 mils, and quite surprisingly, the zirconia was then built up toform a zirconia layer of about 100 mils (layers of as much as 1/4 inchzirconia have been achieved by us by this method). The formed compositewas thermally cycled wherein the zirconia face was subjected to 2900° F.and the metal base was exposed to air at ambient temperature over aseries of cycles without any appreciable separation of the ceramiczirconia from the metal.

EXAMPLE II

According to the teachings of U.S. Pat. No. 3,127,668 a felt web madefrom one-half inch kinked 4 mil wire of Hastelloy X metal alloy wassintered for 10 hours in a furnace vacuum of 10⁻⁵ torr and at atemperature of 2175° F. The web produced has an approximate 20% density.A metal base of Hastelloy X alloy was brazed to the sintered web byexposing the web and the base metal to 2150° in a vacuum furnace forabout 10 minutes. A ceramic composite of ceramic material and staplecard wires was prepared by providing a bed of upstanding 12 mil thickstaple wires having a 3/8 inch U-shape projecting through a porousfabric base that holds the staples in a semi-upright position. A waterbased slurry formed of alumino-silicate mineral fibers having diametersranging from 8 microns to 80 microns were mixed with a low expansionglass powder (the powder having a size where it will pass through a 325mesh screen, the powders having a diameter up to 44 microns); the slurryhaving a composition of 50% aluminum silicate and 50% glass by weightmixed with 50% by volume water. The slurry was deposited on the fabricover and surrounding the upright metal staples and held in place by anextermal holding container. This slurry-staple composite was sintered at2300° F. for 2 hours in a furnace purged with argon to permit the glassto melt reacting with the alumino-silicate and at the same time form amatrix around the alumino-silicate to eliminate any free quartz--thefinal product is the staple impregnated low expansion ceramic whereinthe ceramic is mullite and glass combination (mullite-3Al₂ O₃.2SiO₂).

In a felted slurry mixture, alumino-silicate fibers, having a diameterof approximately 8 microns and a length of 1/8 of an inch andconstituting 98% by weight, were combined with 2% alumino-borosilicateglass fibers, also having a diameter of approximately 8 microns and alength of about 1/8 of an inch, and mixed together with 450 parts ofwater to one part of solid. This mixture was suctioned deposited to forma porous ceramic felt that was compressed to about 40% density. Thedensified ceramic felt was sintered at 2900° F. in an air atmosphere forabout 4 hours wherein the glass fiber melted and reacted tying up thefree quartz resulting in a combination of mullite plus glass; theresulting structure being about 1/8 of an inch thick and 65% dense. Thisceramic material was attached to one side of the staple ceramiccomposite by a low expansion glass powder such as, in wt. percent, 80.5SiO₂ -12.9 B₂ O₃ -3.8 Na₂ O-2.2 Al₂ O₃ -0.4 K₂ O and at the same timethe free surface of the metal fiber web was spot brazed using NicrobrazLM (trademark of Wall Colomony Company) to the other side of thestaple-ceramic composite by placing in a furnace for 10 minutes at 2150°F. in an argon atmosphere. The finally formed composite was thermallycycled to 1800° F. and cooled to ambient. At the end of a 30 cycleperiod the ceramic had not cracked and the interface had maintained itsstructural integrity.

EXAMPLE III

According to the teachings of U.S. Pat. No. 3,127,668, a web made fromone half inch kinked 5 mil wire of FeCrAlSi (Hoskins-875) metal alloywas sintered 9 hours in a furnace vacuum of 10⁻⁵ torr and at atemperature of 2175° F. The web produced has an approximate density of30%. A metal base of high temperature cobalt base alloy was brazed tothe sintered web by exposing the web, braze alloy, and base metal to2150° F. in a vacuum furnace for 10 minutes. A mixture of yttriastabilized zirconia and graphite powders (70-30 by volume, respectively)was plasma sprayed onto the exposed web surface. The sprayed compositewas subsequently exposed to 1700° F. for 15 hrs in air. The burned-offsample had a ceramic layer which was noticeably more porous thangraphite-free zirconia sprayed as described as example I. A secondattached web was plasma sprayed coated with a layer of pure yttriastabilized zirconia and without stopping was then coated with a mixtureof 70 vol. percent yttria stabilized zirconia and 30 vol. percentgraphite. After the graphite was burned off, it was obvious that thelayer near the web was of higher density and, therefore, stronger thanthe outer layer which contained the graphite. Density, as well asstrength, can be controlled by controlling the volume fraction ofgraphite or other sacrificial material.

Examples I, II, and III correspond to three of the embodiments describedherein. It is contemplated that those skilled in the art will thoroughlyunderstand that it is possible to change the composition of the ceramicmaterial, substitute different metal alloys for both the metallic baseand metal interface. It will be recognized that the invention provides avery effective method of absorbing thermal strains in a ceramic-metallaminate composition structure by providing a low modulous resilient,low density metal fiber mat interface joined to the metal base and theceramic.

Other technically significant applications of the embodiments of thisinvention can include gas turbine shrouds, burner cans, vane end walls,magnetohydrodynamics reactors, nuclear fusion reactors, and coatings forpistons and cylinders in diesel and gasoline engines.

Although specific embodiments of the invention have been described manymodifications and changes may be made to the materials, configurationsand methods of making the ceramic-metal composite without departing fromthe spirit and the scope of the invention as defined in the appendedclaims.

What is claimed is:
 1. An insulating coating for a piston of a gasolineor diesel internal combustion engine comprising:(1) a piston body havinga top portion that can be exposed to combustion gases; (2) a compositemember secured to the top portion of the piston, the composite membercomprising a flexible low modulus, low density dimensionally stablemetallic web structure having two opposite faces and a first metallicmember having a preselected thermal coefficient of expansion secured toone face of the metallic web structure; the other face of the metallicweb structure secured to the top portion of the piston which is made ofa material with a thermal coefficient of expansion different than thefirst metal member; the composite member insulating the top portion ofthe piston from the combustion gases with the web structure absorbingthe effect of the different thermal expansions of the top portion of thepiston and the first metal member that is in direct contact with theexpansion gases.
 2. An insulating coating of a piston of a gasoline ordiesel internal combustion engine comprising:(1) a piston body having atop portion that can be exposed to combustion gases; (2) a compositemember secured to the top portion of the piston, the composite membercomprising a flexible low modulus, low density dimensionally stablemetallic web structure having two opposite faces and a first memberhaving a preselected thermal coefficient of expansion secured to oneface of the metallic web structure; the other face of the metallic webstructure secured to the top portion of the piston which is made of amaterial with a thermal coefficient of expansion different than thefirst member; the composite member insulating the top portion of thepiston from the combustion gases with the web structure absorbing theeffect of the different thermal expansions of the top portion of thepiston and the first member that is in direct contact with the expansiongases.
 3. The piston of claim 2 wherein the first member is metal and issecured to the web structure by means of metallic bonding.
 4. The pistonof claim 3 wherein the bonding may be selected from brazing, welding,sintering, or diffusion bonding.
 5. The piston of claim 2 wherein thefirst member is ceramic.
 6. The piston of claim 2 wherein the firstmember is a ceramic-metal composition.
 7. The piston of claim 6 whereinthe first member is a cermet.
 8. An insulating coating of a piston of agasoline or diesel internal combustion engine comprising:(1) a pistonbody having a top portion that can be exposed to combustion gases; (2) acomposite member secured to the top portion of the piston, the compositemember comprising a flexible low modulus, low density dimensionallystable metallic web structure having two opposite faces and a firstceramic member having a preselected thermal coefficient of expansionsecured to one face of the metallic web structure; the other face of themetallic web structure secured to the top portion of the piston which ismade of a material with a thermal coefficient of expansion differentthan the first ceramic member; the composite member insulating the topportion of the piston from the combustion gases with the web structureabsorbing the effect of the different thermal expansions of the topportion of the piston and the first metal member that is in directcontact with the expansion gases.
 9. The piston of claim 8 wherein themetallic web is partially imbedded into the ceramic member.
 10. Thepiston of claim 8 wherein the ceramic member is a laminate structurehaving layers of ceramic and metal materials.